On Sep 17, 2009, at 11:25 AM, esawdust wrote:
I've been trying to understand the as.Date functionality and I have
a date
and time stamp field that looks like this:
"Tue Sep 15 09:22:09 -0600 2009"
and I need to turn it into an R Date object for analysis.
Simple date conversions I have down, no problem:
adate = c("7/30/1959")
as.Date(adate,"%m/%d/%Y")
[1] "1959-07-30"
But when it comes to the type of date/time string format I have
above, I
can't figure out a format string that will work.
The timezone offset is one part that causes problems. Building up
to a
working format string for the full time stamp string, I can make it
as far
as:
adate = c("Tue Sep 15 09:22:09 -0600 2009")
as.Date(adate,format="%a %b %d %H:%M:%S")
[1] "2009-09-15"
(apparently year defaults to current year when it's not specified in
the
format string). Because the Year comes after the timezone offset, I
have to
deal with the timezone offset in the format string.
But when I get to the timezone offset value I can't use "%z" or "%Z"
because
those are "output only"
as.Date(adate,format="%a %b %d %H:%M:%S %z %Y")
[1] NA
You are confusing R Date objects with the the date-time classes. I
don't think Date classes objects even allow TZ offets.
?DateTimeClasses
> as.POSIXct(as.Date(adate,"%m/%d/%Y"), origin="1960-01-01", tz="GMT")
[1] "1959-07-29 20:00:00 EDT"'
Notice that my TZ (GMT -4) was used. so it was still the prior day in
New England.
I'm close, but can't incorporate the timezone offset field in the
date/time
stamp string.
What am I missing? I suppose one workaround is to split the date/
time
string into its component parts, reassemble it into a string as.Date
can
deal with, but that seems to defeat one of the purposes of as.Date's
format
capability.
Any advice for how to translate a "Tue Sep 15 09:22:09 -0600 2009"
into an R
Date object?
Landon
--
David Winsemius, MD
Heritage Laboratories
West Hartford, CT
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